THIS invention relates to data communication systems, more particularly to interrogation systems such as electronic identification systems.
Radio frequency (RF) electronic identification systems including an interrogator or reader and a plurality of passive transponders are well known in the art. In use, the reader energizes the transponders by transmitting an interrogation signal. Each energized transponder automatically responds with a response signal, normally including an identification code characteristic of the transponder. The response signals are received by the reader and the data is read. The data may be utilized to identify transponders and goods associated with the transponders and/or to count the transponders and/or goods. However, when a plurality of transponders are energized simultaneously, it may well happen that the response signals overlap in time, that the transmissions clash and that the data in the response signals is lost.
Accordingly, systems of the aforementioned kind provide for anti-clash or anti-collision measures or protocols. One way of reducing the effect of collisions is to make use of two-way communications. In a typical system employing two way communications, once the data of a transponder has been read, the reader transmits an acknowledgement signal. This acknowledgement signal causes the transponder just read to enter a sleep mode wherein it no longer responds to the interrogation signal, even while still being energized. This switching of a transponder just read to a sleep mode, reduces the transponder population not yet read and hence reduces the probabilities of collisions. The major disadvantage of this protocol is that it requires a larger communications channel bandwidth. Accordingly, it is not suitable for many narrow band applications.
In so-called free-running systems (these are systems where no acknowledgement signal is transmitted) it has been proposed, for example in SA Patent 95/9519, to configure the transponders such as to retransmit the reply signals at random inter-transmission intervals. This gives a statistical probability that transponders would sometimes transmit at a time when no other transponder is transmitting, thereby enabling the reader eventually to receive a reply signal successfully from each transponder. However, if a large number of transponders are present, the probability of a successful transmission is reduced and the time required to receive the transmissions from all the transmitters becomes larger, i.e. the communication channel becomes congested. If the number of transponders is increased too much, the probability of a successful transmission may become so low, that the time required to receive a transmission from each of the transponders becomes unpractically long (even infinitely long). The communications channel can then be said to be saturated. With these systems and if the maximum inter-transmission interval is made shorter, the communications channel may saturate with a relatively small number of transponders, but on the other hand the time required successfully to read a small number of transponders would be short. Furthermore, if the maximum inter-transmission interval is made longer to accommodate a larger transponder population, the total time to read the transponders (even a small number of transponders), may be too long for many applications. In practice this means that a specific transponder can only be used in an application where its maximum inter-transmission interval is compatible with the number of transponders that would be present and the time required successfully to read the transponders.